Method and apparatus for code group identification and frame synchronization in DS/CDMA systems

ABSTRACT

The present invention provides an apparatus and method for code group identification and frame synchronization for cell searching used in wide-band DS-CDMA cellular systems. This method characterizes each secondary synchronization code sequence (SSCS) with a corresponding theoretical frequency sequence, which represents the occurrence times of CS 1  to CS 16  in a corresponding SSCS. Thus, 64 secondary synchronization code sequences corresponding to 64 code groups defined in DS-CDMA systems also corresponds to 64 theoretical frequency sequences. By characterizing the SSCS transmitted by a base station, a real frequency sequence can be generated. Comparing the real frequency sequence with the 64 theoretical frequency sequences, one can determine one or two candidate code groups, which may be employed by the base station. Finally, one can compare the SSCS transmitted by the base station with all the possible SSCSs corresponding to the candidate code groups to determine a specific code group and a frame boundary for the base station.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to an apparatus andmethod for code group identification and frame synchronization used indirect-sequence code division multiple access (DS-CDMA) communicationsystems, such as wide-band CDMA systems and 3^(rd) generationpartnership project (3GPP) system.

[0003] 2. Description of the Related Art

[0004] Currently, DS-CDMA cellular systems are classified as inter-cellsynchronous systems with precise inter-cell synchronization andasynchronous systems without it. For inter-cell synchronous systems, anidentical long code is assigned to each base station, but with adifferent time offset. The initial cell search can be executed byperforming timing acquisition of the long code. The search for aperipheral cell on hand-offs can be carried out quickly because themobile station can receive the offset information of the long code forthe peripheral base station from the current base station. However, eachbase station requires a highly-time consistent apparatus, such as theglobal position system (GPS) and rubidium backup oscillators. Moreover,it is difficult to deploy GPS in basements or other locations RF signalscannot easily reach.

[0005] In asynchronous systems such as wide-band CDMA and 3GPP, eachbase station adopts two synchronization channels as shown in FIG. 1,such that a mobile terminal can establish the link and will not loseconnection on hand-offs by acquiring the synchronization codestransmitted in synchronization channels. The first synchronizationchannel (primary synchronization channel, hereinafter PSCH) consists ofan unmodulated primary synchronization code (denoted as C_(psc)) withlength of 256 chips transmitted once every slot. C_(psc) is the same forall base stations. This code is periodically transmitted such that it istime-aligned with the slot boundary of downlink channels as illustratedin FIG. l. The second synchronization channel (secondary synchronizationchannel, hereinafter SSCH) consists of a sequence of 15 unmodulatedsecondary synchronization codes (C_(ssc) ^(i,1) to C_(ssc) ^(i,15))repeatedly transmitted in parallel with C_(psc) in the PSCH. 15secondary synchronization codes are sequentially transmitted once everyframe. Each secondary synchronization code is chosen from a set of 16different orthogonal codes of length 256 chips. This sequence on theSSCH corresponds to one of 64 different code groups the base stationdownlink scrambling code belongs to. The code allocation for a basestation is shown in FIG. 2. These 64 sequences are constructed such thattheir cyclic-shifts are unique. In other words, if the count ofcyclic-shifting is 0 to 14, all 960 (=64*15) possible sequencesgenerated by cyclic-shifting the 64 sequences are different from eachother. Base upon this property, cell search algorithms can be developedto uniquely determine both the code group and the frame timing.

[0006] During the initial cell search for the wide-band CDMA systemproposed by 3GPP, a mobile station searches for the base station towhich it has a lowest path loss. It then determines the downlinkscrambling code and frame synchronization of the base station. Thisinitial cell search is typically carried out in three steps:

[0007] Step 1: Slot Synchronization

[0008] During the first step of the initial cell search procedure, themobile station searches for the base station to which it has lowest pathloss via the primary synchronization code transmitted through the PSCH.This is typically done with a single matched filter matching to theprimary synchronization code. Since the primary synchronization code iscommon to all the base stations, the power of the output signal of thematched filter should have peaks for each ray of each base stationwithin a receivable range. The strongest peak corresponds to the moststable base station for linking. Detecting the position of the strongestpeak yields the timing and the slot length that the strongest basestation modulates. That is, this procedure causes the mobile station toacquire slot synchronization to the strongest base station.

[0009] Step 2: Frame Synchronization and Code-Group Identification

[0010] During the second step of the cell search procedure, the mobilestation utilizes the secondary synchronization code in the SSCH to findthe frame synchronization and the code group of the cell found in thefirst step. Since the secondary synchronization code is transmitted inparallel with the primary synchronization code, the position of thesecondary synchronization code can be found after the first step. Thereceived signal at the positions of the secondary synchronization codeis consequently correlated with all possible secondary synchronizationcodes for code identification. 15 consecutive codes received andidentified within one frame construct a received sequence. Because thecycle shifts of the 64 sequences corresponding to 64 code groups areunique, by correlating the received sequence with the 960 possiblesequences, the code group for the strongest base station as well as theframe synchronization is determined.

[0011] Step 3: Scrambling-Code Identification

[0012] During the last step of the cell search procedure, the mobileterminal determines the exact primary scrambling code used by the foundbase station. The primary scrambling code is typically identifiedthrough symbol-to-symbol correlation over the Common Pilot Channel(hereinafter CPICH) with all codes within the code group identified inthe second step. After the primary scrambling code has been identified,the Primary Common Control Physical Channel (hereinafter PCCPCH) can bedetected. Then the system and cell specific information can be read.

[0013] In sum, the main tasks of the initial cell search procedure areto (1) search for a cell with the strongest received power, (2)determine frame synchronization and code group, and (3) determine thedown-link scrambling code.

[0014] An intuitive implementation for code group identification andframe synchronization are illustrated in FIG. 3. R_(I)(m) and R_(q)(m)are signals demodulated by QPSK (quaternary phase shift keying) withphase difference of π/2. 16 correlators 2101˜2116 correlate R_(I)(m) andR_(q)(m) received in a slot with different correlation co-efficiencies,respectively, to determine the similarities for the representation ofR_(I)(m) and R_(q)(m) to 16 orthogonal codes CS₁ to CS₁₆. A codelocation table 26 records the 16 code groups in FIG. 2, totaling 960secondary synchronization codes. The code location table 26 sequentiallyprovides the stored secondary synchronization codes. For example, in afirst time slot, the code location table 26 sends out the 960 secondarysynchronization codes from the codes in column 1 to the codes in column15. In a next time slot, the code location table 26 sends out the 960secondary synchronization codes from the codes in column 2 to the codesin column 15 and back to codes in column 1, etc. It is emphasized thatthe output sequence from the code location table 26 is slot-dependent.The 16-to-1 multiplexor passes one of the 16 similarities from thecorrelators, according to the secondary synchronous code it received, toone of the 960 shift registers 24. 960 shift registers 24 store the 16similarities within a slot, and accumulate the similarities from slot toslot. After accumulating within a frame (15 slots), a maximum finder 25can find one of the 960 shift registers 24 having a highest similarityand determines the frame boundary and the code group.

SUMMARY OF THE INVENTION

[0015] Therefore, an object of the present invention is to provide anapparatus and method for efficient code group identification and framesynchronization.

[0016] To achieve the aforementioned purpose, the present inventionprovides a method for code group identification and framesynchronization. The first step of the method is providing secondarysynchronization code sequences SSCS₁, SSCS₂, . . . , SSCSK with lengthof L codes. The secondary synchronization code sequences SSCS₁, SSCS₂, .. . , SSCSK are corresponding to code groups GCS₁, GCS₂, . . . , GCSkand constructed of CS₁, CS₂,. . . , CS_(N). Then, the method accordingto the present invention has a step of providing K theoretical frequencysequences with length of N elements. Each theoretical frequency sequencerepresents the theoretical-occurrence times of CS₁, CS₂, . . . , CS_(N)in a corresponding secondary synchronization code sequence. Then, themethod has a step of sensing and recording, consecutively, secondarysynchronization codes from a base station to form a received codesequence with length of L codes, each code in the received code sequencebeing selected from CS₁, CS2 ₁, . . . , CS_(N). The following step iscounting the occurrence times of CS₁, CS₂, . . . , CS_(N) in thereceived code sequence to form a testing sequence with length of Nelements. Then the method has a step of comparing, one-by-one, thetesting sequence with the K theoretical frequency sequences to retrievea candidate code group, wherein the theoretical frequency sequencecorresponding to the candidate code group is most similar to the testingsequence. The candidate code group corresponds to a candidate secondarysynchronization code sequence. Then the method has a step of comparingthe received code sequence with all possible sequences generated bycycle-shifting the candidate secondary synchronization code sequence toretrieve a most-likely code sequence which is most similar to thereceived code sequence. The final step of the method is determining acode group and a frame boundary according to the most-likely sequence.

[0017] Another aspect of the present invention is providing an apparatusfor code group identification and frame synchronization. The apparatusaccording to the present invention is applied to a DS-CDMA communicationsystem, and comprises a first memory set, a second memory set, areceiver, a summation unit, a group searcher and a frame alignment unit.The first memory set stores secondary synchronization code sequencesSSCS₁, SSCS₂, . . . , SSCS_(K) with length of L codes. The secondarysynchronization code sequences SSCS₁, SSCS₂, . . . , SSCS_(K) correspondto code groups GCS₁, GCS₂, . . . , GCS_(k) and are constructed of CS₁,CS₂, . . . , CS_(N). The second memory set records K theoreticalfrequency sequences with length of N elements, each theoreticalfrequency sequence representing the theoretical-occurrence times of CS₁,CS₂, . . . , CS_(N) in a corresponding secondary synchronization codesequence. The receiver receives and extracts secondary synchronizationcodes transmitted from a base station in an observation frame to form areceived code sequence with length of L codes. Each code in the receivedcode sequence is selected from CS₁, CS₂, . . . , CS_(N). The summationunit counts the occurrence times of CS₁, CS₂, . . . , CS_(N) in thereceived code sequence to form a testing sequence with length of Nelements. The group searcher compares, one-by-one, the testing sequencewith the K theoretical frequency sequences to retrieve a candidate codegroup, wherein the candidate code group corresponds to a most-likelytheoretical frequency sequence which is most similar to the testingsequence. The candidate code group also corresponds to a candidatesecondary synchronization code sequence. The frame alignment unitcompares the received code sequence with all possible sequencesgenerated by cycle-shifting the candidate secondary synchronization codesequence to retrieve a most-likely code sequence which is most similarto the received code sequence. Thereby, the frame alignment unitdetermines a code group and a frame boundary according to themost-likely sequence.

[0018] The advantage of the present invention is it increases the speedfor code group identification and frame synchronization. The candidatecode group can be quickly obtained by comparing the testing sequencewith the K theoretical frequency sequences. Thereby, framesynchronization can be easily achieved by comparing the received codesequence with all the possible secondary synchronization code sequencesrelative to the candidate code group.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The drawings referred to herein will be understood as not beingdrawn to scale except if specially noted, the emphasis instead beingplaced upon illustrating the principles of the present invention. In theaccompanying drawings:

[0020]FIG. 1 illustrates the relationship of the primary and thesecondary synchronization channels;

[0021]FIG. 2 shows the code allocation for a base station;

[0022]FIG. 3 demonstrates an intuitive implementation for code groupidentification and frame synchronization;

[0023]FIG. 4 is a provisional diagram for a table illustrating theoccurrence times of the secondary synchronization codes (CS₁ to CS₁₆) ina relative SSCS;

[0024]FIG. 5 is a flow chart for a mobile terminal to identify codegroup and synchronize time frame according to the present invention;

[0025]FIG. 6 shows an apparatus according to the present invention;

[0026]FIG. 7 illustrates an implementation for the summation bank inFIG. 6;

[0027]FIG. 8 illustrates a schematic diagram of the group finder in FIG.6;

[0028]FIG. 9 illustrates a group decision table generated bytransforming the table in FIG. 4; and

[0029]FIG. 10 illustrates a schematic diagram of the frame alignmentunit in FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0030] The present invention provides a method and apparatus for codegroup identification and frame synchronization. The spirit of thepresent invention is characterizing the 64 secondary synchronizationcode sequences (SSCSs) corresponding to the 64 code groups with 64features prior to executing a comparison. In the embodiment below, eachfeature for a corresponding SSCS is the representation of the occurrencetimes of CS₁to CS₁₆ in the corresponding SSCS. Such a feature of theSSCS transmitted by a base station can also be recognized to excludemost of the SSCSs of the 64 code groups from comparison. That means,very few of the 64 SSCSs having the similar feature with that of thebase station are needed to execute the comparison for codeidentification and frame synchronization. Therefore, the speed foridentification and synchronization can be increased.

[0031]FIG. 4 is a provisional diagram for a table illustrating theoccurrence times of the secondary synchronization codes (CS₁ to CS₁₆) ina relative SSCS. The table in FIG. 4 is generated from the table in FIG.2. Every row in the table of FIG. 4 shows the occurrence times of CS₁ toCS₁₆ in the SSCS of a corresponding code group. It is illustrated inFIG. 2 that CS₁ and CS₂ each appear two times and CS₃ doesn't appear inthe SSCS of code group 1. Thus the theoretical frequency sequence forcode group 1, which is shown in the row 1 of FIG. 4, is (2, 2, 0, . . .). Following the same concept, each theoretical frequency sequence foreach code group can be determined. The result is shown in FIG. 4. FIG. 4also illustrates one important point, which is that, at most, twotheoretical frequency sequences among these 64 theoretical frequencysequences are identical. Thus, if one can find out a real frequencysequence relative to the SSCS transmitted by a base station, such a realfrequency sequence can be used as a criterion to filter out the codegroups having totally different theoretical frequency sequences as thereal frequency sequence.

[0032]FIG. 5 is a flow chart for a mobile terminal to identify codegroup and synchronize time frame according to the present invention.First, a mobile terminal, e.g. a cellular phone, receives and recordsthe secondary synchronization codes transmitted in an observation frameby a base station (60). These codes form a received SSCS with length of15 codes (68). During forming the received SSCS, the occurrence times ofCS₁ to CS₁₆ can also be determined to form a testing sequence withlength of 16 elements (62). From FIG. 4, it is known that each secondarysynchronization code happens in an SSCS no more than two times. Thus, ifthe testing sequence has an element with a value greater than 2 (Yesroute in symbol 64), another observation frame are required to generatea newborn receive SSCS since there are too many errors in the originalreceive SSCS. When all elements in the testing sequence have values lessthan or equal to 2 (No route in symbol 64), by comparing the testingsequence with the table shown in FIG. 4, at least one most-likelytheoretical frequency sequence, which is most similar to the testingsequence, can be determined (66). A most-likely theoretical frequencysequence corresponds to a candidate code group. Then, by comparing thereceived SSCS with all the possible SSCSs generated by cycle shiftingthe SSCS relative to the candidate code group(s), a most-likely SSCSamong those cycle-shifted SSCSs can be determined (70). If there aremore than 2 codes being different between the most-likely SSCS and thereceived SSCS (Yes in symbol 72), it means there are too many errors, sothat an overall re-execution of this method is conducted. If thedifferences between the most-likely SSCS and the received SSCS are notmore than two codes (No in symbol 72), the code group and the frameboundary can be determined by the most-likely SSCS.

[0033]FIG. 6 shows an apparatus according to the present invention. A RF(radio frequency) receiver 76 transforms spread spectrum signal receivedby an antenna to intermediate frequency signals. A demodulator 80demodulates and samples the intermediate frequency signals, and outputsin-phase samples R_(i)(t) and quadrature-phase samples R_(q)(t) Acorrelator controller 78 supplies the correlation coefficients to amatched correlator bank, in which a predefined number of correlators8201-8216 correspond to the secondary synchronization codes CS₁ to CS₁₆.The correlator controller 78 enables the correlators 8201-8216 tocorrelate in-phase samples R_(i)(t) and quadrature-phase samplesR_(q)(t) at the start-up position of each slot for 256 chip period, andthe correlators 8201-8216 will produce the output power level,respectively. Each output power level represents the similarity level toa corresponding secondary synchronization code for the in-phase samplesR_(i)(t) and quadrature-phase samples R_(q)(t) within a slot. Theseoutput power levels are then sent into a frame-wise accumulator bankwith 16 frame-wise accumulators 8401-8416. Each frame-wise accumulator84 n (n is an integer within [1,16]) consists of an adder 40 n and ashift register 42 n of fifteen words, as shown in FIG. 6. Since a framehas fifteen slots in the synchronization channel, the output powerlevels for a given slot in consecutive observation frames can be summedtogether frame by frame for reliable decision of code group.

[0034] After the frame-wise accumulator bank performs accumulation overa plurality of consecutive observation frames, the frame-wiseaccumulator 84 n has fifteen summed output power levels in the shiftregister 42 n. These contents of the shift register 42 n are in parallelshifted out to a maximum selector 86 one by one. The maximum selector 86then determines which has the maximum power among 16 accumulated powerlevels C1 to C16. Assuming that the content of the shift register 42 nat the sth word has the largest accumulated power level among those ofthe shift registers 4201-4216 at sth word (s is an integer within [1,15]), the output signal r_(n)(s) is 1, while the other output signalsr_(m)(s), m≠n, are set as 0. At the same time, CS_(n)(s), themost-likely secondary synchronization code in a given slot, is sent to ashift register 90 of fifteen words. Therefore, the shift register 90restores 15 most-likely secondary synchronization codes, transmitted bya base station and detected within an observation frame, to construct adetected code sequence.

[0035] The output signal r_(n)(s) is sent to a summation bank 88 forcounting the occurrence times of each secondary synchronization code.Since the output signal r_(n)(S) from the maximum selector 86 is either1 or 0, the summation bank 88 can be implemented by 2-bit counters9101-9116. Each counter is either enabled for incrementing or disabledfor holding the present count. The implementation for the summation bank88 is shown in FIG. 7. The process keeps going for fifteen iterationsuntil the content of each shift register are serially shifted out. Afterthe fifteen contents of each shift register 42 n are all passed throughthe summation bank 88, the count of the nth counter 91 n in thesummation bank 88 represents the occurrence times when the matchedcorrelator 82 n has the maximal output power level within oneobservation frame. That is, if the communication environment wasnoiseless and interference-free, the contents of the nth counter 91 nexactly represent the occurrence times of the secondary synchronizationcode CS_(n) used within one frame. Utilizing this property, a mobileterminal can determine the code group that is used by the base stationsynchronized to the mobile terminal. As shown in FIG. 4, the occurrencetime for each secondary synchronization code CS_(n) within anobservation frame is no more than 2. Therefore, 16 “and gates” 9301-9316and “or gate” 97 are used for resetting all the system when one of the16 occurrence times respectively for the 16 secondary synchronizationcodes is higher than 2. The contents of the counters 9101-9116 areencoded as

[0036] 0→00,

[0037] 1→01,

[0038] 2→11,

[0039] and collected into a sequence, called a test sequence. The testsequence consists of C_(1,LSB), C_(1,MSB), . . . , C_(16,LSB),C_(16,MSB). Following the above encoding rule, one can obtain the groupdecision table shown in FIG. 9 by transforming the table in FIG. 4. Thegroup decision table is saved in the memory 92 in FIG. 6. The kth row ofthe group decision table shown in FIG. 9 is an ideal sequence T_(k) (kis an integer within [1, 64]), which represents the occurrence times ofCS₁ to CS₁₆ for the kth code group within one frame at an ideal (nonoise) condition.

[0040] By comparing the test sequence with the ideal sequences T₁ to T₆₄in the group decision table row by row, the code group number can bediscriminated. At most two ideal sequences yield the best similarity andare called candidate sequences. These two candidate sequences, forexample, corresponding to the two possible code groups, may be used bythe base station synchronized to the mobile terminal. A group finder 94shown in FIG. 6 executes such code group discrimination. FIG. 8illustrates a schematic diagram of the group finder in FIG. 6. 322-input XOR (exclusive or) logic gates 8101-8132 are used to in parallelto compare the bits in the test sequence with those in an ideal sequenceTk bit by bit. A 2-input XOR logic gate 81 h (h is an integer within [1,32]) has an output signal of “0” when its two inputs are of the samevalue, and “1” for other conditions. A 32-input adder 83 accumulates theoutput signals of the 2-input XOR logic gates 8101-8132 to obtain asimilarity value S_(k) corresponding to an ideal sequence T_(k). 64similarity values S₁ to S₆₄ are sequentially stored in a shift register42. The smaller the value of S_(k) the more similarity between the testsequence and the ideal sequence T_(k). A minimum finder 85 finds out theleast two similar values among S₁ to S₆₄ and outputs two candidate codegroup numbers.

[0041] The code allocation table 96 in FIG. 6 stores the 64 secondarysynchronization code sequences corresponding to the 64 code groups, andoutputs two candidate SSCSs corresponding to the two candidate codegroup numbers obtained from the minimum finder 85. A frame alignmentunit 98 compares the detected code sequence in the shift register 90with all possible secondary synchronization code sequences generated bycycle-shifting the two candidate SSCSs to determine the exact code groupnumber and the accurate frame boundary. FIG. 10 illustrates a schematicdiagram of the frame alignment unit in FIG. 6. The code indexes of thedetected code sequence in the shift register 90 are in parallel shiftedout to the shift register 104. One possible code sequence formed bycycle shifting one candidate SSCS is stored in the shift register 106.The code indexes at the same positions of the shift registers 104 and106 are in parallel shifted out as two inputs of the comparator 108. Theoutput of the comparator 108 has an output signal of “0” when the twoinput are of the same value, and “1” for other conditions. A register108 and an adder 112 summarize the 15 output results and output acorresponding summation to a shift register 114. A SSCS relative to acandidate code group can generate 15 possible cycle-shifted SSCSs, towhich 15 summations V₁ to V₁₅ stored in shift register 114 arecorresponding. If there are two candidate code groups, the shiftregister 114 must store 30 summations since there are 30 possiblecycle-shifted SSCSs. The smaller a summation, the more similar between acorresponding cycle-shifted SSCS and the detected code sequence. Theminimum selector 116 retrieves the smallest among the summations in theshift register 114 to determine the code group employed by thesynchronized base state and the frame boundary.

[0042] By employing the method provided by the present invention, thehuge task of comparing the detected code sequence with the 960 possiblecycle-shifted SSCSs corresponding to the 64 code groups can be avoid.There are at most two candidate code groups after comparing theoccurrence times of the secondary synchronization codes for the detectedcode sequence with those for the 64 SSCSs. Therefore, fewer tasks areneeded to determine code group and frame synchronization by takingreference of the two candidate code groups. Thereby, the processing ratecan be speed up.

[0043] While the invention has been described by way of example and interms of the preferred embodiment, it is to be understood that theinvention is not limited to the disclosed embodiment. On the contrary,it is intended to cover various modifications and similar arrangementsas would be apparent to those skilled in the art. Similarly, any processsteps described herein may be interchangeable with other steps in orderto achieve the same result. Therefore, the scope of the appended claimsshould be accorded the broadest interpretation so as to encompass allsuch modifications and similar arrangements, which is defined by thefollowing claims and their equivalents.

What is claimed is:
 1. A method for code group identification and framesynchronization, comprising the following steps: providing secondarysynchronization code sequences SSCS₁, SSCS₂, . . . , SSCS_(K) withlength of L codes, secondary synchronization code sequences SSCS₁,SSCS₂, . . . , SSCS_(K) corresponding to code groups GCS₁, GCS₂, . . . ,GCS_(k) and constructed of CS₁, CS₂, . . . , CS_(N); providing Ktheoretical frequency sequences with length of N elements, eachtheoretical frequency sequence representing the theoretical-occurrencetimes of CS₁, CS₂, . . . , CS_(N) in a corresponding secondarysynchronization code sequence; sensing and recording, consecutively,secondary synchronization codes from a base station to form a receivedcode sequence with length of L codes, each code in the received codesequence being selected from CS₁, CS₂, . . . , CS_(N); counting theoccurrence times of CS₁, CS₂, . . . , CS_(N) in the received codesequence to form a testing sequence with length of N elements;comparing, one-by-one, the testing sequence with the K theoreticalfrequency sequences to retrieve a candidate code group, wherein thetheoretical frequency sequence corresponding to the candidate code groupis most similar to the testing sequence, the candidate code groupcorresponding to a candidate secondary synchronization code sequence;comparing the received code sequence with all possible sequencesgenerated by cycle-shifting the candidate secondary synchronization codesequence to retrieve a most-likely code sequence which is most similarto the received code sequence; and determining a code group and a frameboundary according to the most-likely sequence.
 2. The method as claimedin claim 1, further comprising a step of consecutively re-sensing andrecording secondary synchronization codes from the base station toreplace the received code sequence when one of CS₁, CS₂, . . . , CS_(N),in the received code sequence has an occurrence time more than
 2. 3. Themethod as claimed in claim 1, wherein the number of candidate codegroups is not more than
 2. 4. The method as claimed in claim 1, furthercomprising a step of consecutively re-sensing and recording secondarysynchronization codes from the base station to replace the received codesequence when the received code sequence and the most-likely sequencehave different codes at two corresponding places.
 5. The method asclaimed in claim 1, wherein the method is employed in a direct-sequencecode division multiple code access (DS-CDMA) communication system.
 6. Aapparatus for code group identification and frame synchronization,applied for a DS-CDMA communication system, comprising: a first memoryset for storing secondary synchronization code sequences SSCS₁, SSCS₂, .. . , SSCS_(K) with length of L codes, secondary synchronization codesequences SSCS₁, SSCS₂, . . . , SSCS_(K) corresponding to code groupsGCS₁, GCS₂, . . . , GCS_(k) and constructed of CS₁, CS₂, . . . , CS_(N);a second memory set for recording K theoretical frequency sequences withlength of N elements, each theoretical frequency sequence representingthe theoretical-occurrence times of CS₁, CS₂, . . . , CS_(N) in acorresponding secondary synchronization code sequence; a receiver forreceiving and extracting secondary synchronization codes transmittedfrom a base station in an observation frame to form a received codesequence with length of L codes, each code in the received code sequencebeing selected from CS₁, CS₂, . . . , CS_(N); a summation unit forcounting the occurrence times of CS₁, CS₂, . . . ,CS_(N) in the receivedcode sequence to form a testing sequence with length of N elements; agroup searcher for comparing, one-by-one, the testing sequence with theK theoretical frequency sequences to retrieve a candidate code group,wherein the candidate code group corresponds to a most-likelytheoretical frequency sequence which is most similar to the testingsequence and corresponding to a candidate secondary synchronization codesequence; and a frame alignment unit for comparing the received codesequence with all possible sequences generated by cycle-shifting thecandidate secondary synchronization code sequence to retrieve amost-likely code sequence which is most similar to the received codesequence and determining a code group and a frame boundary according tothe most-likely sequence.
 7. The apparatus as claimed in claim 6,wherein the receiver comprises: a radio frequency (RF) receiver fortransforming spread signals transmitted from the base station tointermediate signals; a demodulator for demodulating and sampling theintermediate signals in each slot to generate an in-phase signal and aquadrature-phase signal; N matched correlators for correlating thein-phase signal and the quadrature-phase signal according to N differentcoefficient sets and respectively generating N output power levels ineach slot, N coefficient sets respectively corresponding to CS₁, C₂, . .. , CS_(N); N frame-wise accumulators, each frame-wise accumulatorperiodically accumulating output power levels from a correspondingmatched correlator with a period of a frame and outputting a summationpower level for each slot; a maximum selector for, in each slot, findinga maximum power level among N summation power levels from N frame-wiseaccumulators and correspondingly outputting a retrieved secondarysynchronization code among CS₁, CS₂, . . . , CS_(N) the retrievedsecondary synchronization code being the most possible secondarysynchronization code received in a corresponding slot; and a recorderfor consecutively recording L retrieved secondary synchronization codesfrom the maximum selector in a frame to form the received code sequence.